The Development of the AlHgator 291 



several layers of cells, so that it resembles to a 

 considerable degree the spinal cord of the same 

 region. In the narrow space between the spinal 

 cord and the hindgut is seen the notochord (w/), 

 somewhat flattened and relatively and actually 

 smaller than in the preceding figure. A few scat- 

 tered blood-vessels may be seen in the mesoblast at 

 various places. 



A sagittal section of an embryo of this stage, 

 drawn under the same magnification as were the 

 transverse sections, is shown in Figure 13^. The 

 embryo being bent laterally could not be cut by 

 any one plane throughout its entire length, so 

 that only the anterior end is represented in the 

 figure. The amnion (a) may be clearly seen except 

 at certain places where it is closely adherent to 

 the superficial ectoderm. Under the low magni- 

 fication used the superficial ectoderm cannot be 

 distinguished from the ectoderm of the nervous 

 system. The plane of the section being in the 

 anterior end almost exactly median, this part of 

 the central nervous system is seen as the usual 

 retort-shaped cavity, while in the region back 

 of the brain, where the neural canal is narrow, the 

 section passes through the wall of the spinal cord 

 {sc) and does not show the neural canal at all. 

 The wall of the forebrain {fb) is quite thick, espe- 

 cially at the extreme anterior end; the wall of the 

 midbrain (mb), where the marked cranial flexure 

 takes place, is somewhat thinner, and it graduall}^ 



